Aggregate states of matter. Crystalline and amorphous bodies

Features of the internal structure of crystals that distinguish them from non-crystalline (amorphous) bodies are the ordered, periodically repeating arrangement of material particles (atoms, tones, molecules) in space and the symmetry of this arrangement. In this case, this ordering manifests itself at distances significantly exceeding the dimensions of the particles themselves, and is preserved within the entire crystal, i.e. occurs long range order (as opposed to close order - order in the arrangement of particles in the regions closest to a given atom, commensurate with the size of the atoms).

The second feature of crystals is their anisotropy, those. dissimilarity of properties in different directions in the crystal. Anisotropy, or vectorial properties of crystals in different directions, is a consequence of their geometric anisotropy, i.e. differences in material particles and bonds in different directions in the crystal structure.

The third feature of the properties of crystals is their homogeneity, which manifests itself in the fact that any two sections of the crystal have exactly the same properties (in parallel directions).

Crystal - is a solid homogeneous anisotropic body, limited by flat faces that appear on it due to the properties of the body itself, and crystals of the same substance can have different sizes, shapes and number of faces, but the angles between the corresponding faces always remain constant.

Crystalline substances can exist in the form of single crystals or polycrystalline substances. Single crystals are called single crystals found in nature or grown artificially for the needs of science and technology. However, they are much more widespread polycrystalline substances, consisting of many small intergrown single crystals, under normal conditions, differently oriented in relation to each other, the adhesion between which is carried out due to interatomic and intermolecular forces. With such a random orientation, the anisotropy of properties characteristic of single crystals will naturally be absent and, in general, they will be isotropic, i.e. will have the same properties in different directions.

To describe the periodicity in the arrangement of material particles of crystalline phases, the concept of “crystal lattice” is introduced. Crystal cell - a mathematical abstraction characterizing the scheme of three-dimensional periodicity in an infinite system of points (lattice nodes) in space. The entire lattice can be imagined as an infinite system of elementary parallelepipeds, completely filling space due to the endless repetition in three independent directions of one elementary parallelepiped, which is called unit cell. The size of the edges of an elementary parallelepiped and the angles between them are called lattice parameters and are the material constants of each crystalline substance. The unit cell is the smallest part of the crystal, which reflects all the features of its internal structure.

Depending on the type of particles and the predominant type of chemical bond in the crystal, lattices are divided into two large groups: molecular and coordination.

IN molecular lattices nodes contain molecules. Such lattices are characterized by strong intramolecular bonding and weak residual (van der Waals) connection between molecules. Most organic substances belong to compounds with such lattices. They are characterized by low fusibility, high volatility, and low hardness.

In crystals with coordination lattices It is impossible to isolate individual discrete molecules, and the bond forces between a given atom or ion and all its neighbors and in the coordination sphere are approximately the same (in this case, the entire crystal can be considered as one giant molecule). Coordination lattices are characteristic of most inorganic substances, including silicates and other refractory compounds.

Coordination lattices, in turn, can be divided into ionic, atomic (covalent) and metallic. In nodes ionic lattices positive and negative ions are arranged alternately. In nodes atomic (covalent) gratings neutral atoms are located, connected predominantly by covalent bonds. Substances with similar lattices include, for example, diamond, silicon, some carbides, silicides, etc. In nodes metal gratings, characteristic of metals, there are metal ions immersed in an “electron gas”. This lattice structure results in high electrical conductivity, thermal conductivity and plasticity.

An important characteristic of crystal structures is coordination number atoms or ions. The coordination number is the number of particles immediately surrounding a given ion or atom. So, in ion 4- The coordination number of a silicon atom with respect to oxygen is 4.

). In the crystalline state, there is also short-range order, which is characterized by constant coordination. numbers, and chemical lengths. connections. The invariance of short-range order characteristics in the crystalline state leads to the coincidence of structural cells during their translational movement and the formation of three-dimensional periodicity of the structure (see...). Due to its max. orderliness, the crystalline state of the substance is characterized by a minimum. internal energy and is a thermodynamically equilibrium state for given parameters - pressure, t-re, composition (in the case), etc. Strictly speaking, a completely ordered crystalline state cannot really be carried out, approaching it takes place when the t-ry tends to O K (the so-called ideal). Real bodies in a crystalline state always contain a certain number of elements that violate both short- and long-range order. Especially a lot is observed in solid solutions, in which individual particles and their groups statistically occupy different positions. position in space. Due to the three-dimensional periodicity of the atomic structure, the main features are the homogeneity of both properties and edges, which is expressed, in particular, in the fact that under certain conditions the formations take the form of polyhedra (see). Certain holy places on the surface and close to it are significantly different from these holy places inside, in particular due to the violation. The composition and, accordingly, properties change in volume due to the inevitable change in the composition of the medium as it grows. Thus, the homogeneity of the crystalline state, as well as the presence of long-range order, refers to the characteristics of the “ideal” crystalline state. Most bodies in the crystalline state are polycrystalline and represent intergrowths of a large number of small crystallites (grains) - areas of the order of 10 -1 -10 -3 mm in size, irregular in shape and differently oriented. The grains are separated from each other by intercrystalline layers, in which the order of the particles is disturbed. Concentration of impurities also occurs in the intercrystalline layers during the process. Due to the random orientation of the grains, the polycrystalline. body as a whole (volume containing quite a lot of grains) m.b. isotropic, e.g. obtained from crystalline from last . However, usually in the process and especially plastic. texture appears - advantages, crystalline orientation. grains in a certain direction, leading to St. Due to the crystalline state, several components may correspond to a single-component system. fields located in the area of ​​relatively low temperatures and higher. . If there is only one state and the substance does not chemically decompose when the temperature rises, then the state borders on the fields and along the lines and - respectively, and () can be in a metastable (supercooled) state in states, while the crystalline state cannot be in the field or, i.e. crystalline. it cannot be overheated above the temperature or . When heated, certain substances (mesogens) turn into liquid crystalline. condition (see). If there are two or more states on the diagram of a one-component system, these fields border along the line of polymorphic transformations. Crystallic. the substance can be overheated or overcooled below the polymorphic transformation temperature. In this case, the crystalline state of the substance under consideration may be in the field of another crystalline state. modifications and is metastable. While and thanks to the existence of critical. points on a line can be continuously converted into each other, the question of the possibility of continuous mutual transformation. crystalline state and has not been finally resolved. For certain items, the critical value can be assessed. parameters - pressure and temperature, at which D H pl and D Vpl are equal to zero, i.e., crystalline state and are thermodynamically indistinguishable. But it really does turn out like that. was not observed for any species (see). A substance can be transferred from a crystalline state to a disordered state (amorphous or glassy), which does not meet the minimum freedom. energy, not only by change (, t-ry, composition), but also by impact or subtle. Critical particle size, at which it no longer makes sense to talk about the crystalline state, is approximately 1 nm, i.e. of the same order as the size of the unit cell. TO The crystalline state is usually distinguished from other varieties of the solid state (glassy, ​​amorphous) by X-ray diffraction patterns of the substance.
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Spanish literature for the article "CRYSTALLINE STATE": Shaskolskaya M.P., Crystallography, M., 1976; Modern crystallography, ed. B.K. Weinstein. T. I. M., 1979. P. I. Fedorov.

Page "CRYSTALLINE STATE" prepared based on materials.

Solids, as is known, are bodies of a crystalline structure, the patterns of arrangement of atoms in which largely determine their properties. Therefore, it is appropriate to preface the discussion of issues related to reactions in mixtures of solids with a brief summary of modern ideas about the crystalline state of matter.

Particles of a substance in a crystalline state have a stable position and are arranged in an orderly manner, forming a spatial crystal lattice. The structure of this lattice, which is now easily determined by X-ray diffraction, in most cases is closely related to the chemical composition of the substance.

This connection, as the famous crystallographer Fedorov showed back in 1890, can also be observed in a somewhat less clear form in the shape or habit of crystals. Typically, the simpler the chemical composition of a body, the higher the symmetry of its crystals. 50% of elements and about 70% of binary compounds form, for example, cubic crystals, 75-85% of compounds with four to five atoms per molecule form hexagonal and orthorhombic crystals, and about 80% of complex organic compounds form orthorhombic and monoclinic crystals. All this can be explained by the fact that the more homogeneous the components of the crystal lattice, the more orderly they can be located in space.

An interesting pattern that characterizes the relationship between the structure of a crystal and its chemical composition is also the fact that substances related in structure (for example, BaS04, PbS04, SrS04 or CaCO3, MgCO3, ZnCO3, FeCO3, MnC03) crystallize in similar crystalline forms. The similarity of the properties of crystals in the isomorphic series of substances formed in this way corresponds to the similarity of the structure of their crystal lattices.

An important feature of the crystalline state of a substance is its anisotropy, which consists in the difference in the physical properties of a chemically homogeneous crystal in its different directions.

niyah. Anisotropy can be observed in the mechanical, optical, diffusion, thermal and electrical properties of crystalline solids. It manifests itself, by the way, in different rates of crystal growth in different directions, according to which some of its faces develop more than others.

The structural elements that make up the crystal and the interaction forces between them can be different. Accordingly, a distinction is made between ionic, molecular, covalent and metallic lattices. In practice, lattices of various intermediate types are also widely used. Research has established that the bonding in the lattices of many crystalline compounds belongs to the intermediate form and that the nature of the different bonds in a compound of three or more chemical elements is often different. Based on the nature of the prevailing bond forces, they are called ionic, covalent, etc.

In an ionic lattice, characteristic of most salts and typical of inorganic compounds, the interaction forces between its structural elements are mainly electrostatic. Such a lattice is formed by the regular alternation of oppositely charged ions (Fig. 1), interconnected by Coulomb interaction forces.

There are four generally accepted states of matter: solid, liquid, gas and plasma. In addition, the fifth type of aggregative state of matter, discovered with the help of the Large Hadron Collider, was noted in the literature.

In the merchandising of consumer goods, only three states are of practical interest. Any individual element or complex substance can exist sequentially or simultaneously in two or more of these states: water, ice and water vapor can exist at the same temperature and pressure. Solids may be crystalline (have a regularly repeating molecular structure), such as salt and metal; or amorphous, like resin or glass. The molecules of a liquid move, but they are located close to each other, as in solids. In gases, the molecules are so far apart that they move in relatively straight lines until they collide with the walls of the container.

First of all, it should be emphasized once again that gas, liquid and solid are aggregate states of substances, and in this sense there is no insurmountable difference between them: any substance, depending on temperature and pressure, can be in any of the aggregate states. However, there are significant differences between gaseous, liquid and solid bodies.

The essential difference between a gas, on the one hand, and solid and liquid bodies, on the other hand, is that a gas occupies the entire volume of the vessel provided to it, while a liquid or solid placed in a vessel occupies only a very certain volume in it . This is due to the difference in the nature of thermal motion in gases and in solid and liquid bodies.

In solids, atoms can be arranged in space in two ways:

1) an ordered arrangement of atoms, when atoms occupy well-defined places in space. Such substances are called crystalline(Fig. 1.1, a).

The atoms oscillate relative to their average position with a frequency of about 1013 Hz. The amplitude of these oscillations is proportional to temperature;

2) a random arrangement of atoms, when they do not occupy a specific place relative to each other. Such bodies are called amorphous(Fig. 1.1, b).

Rice. 1.1.

Amorphous substances have the formal characteristics of solids, that is, they are able to maintain a constant volume and shape. However, they do not have a specific melting or crystallization point.

Due to the ordered arrangement of atoms of a crystalline substance in space, their centers can be connected by imaginary straight lines. The set of such intersecting lines represents a spatial lattice, which is called a crystal lattice. The outer electron orbits of the atoms are in contact, so the packing density of the atoms in the crystal lattice is very high.

Crystalline solids consist of crystalline grains - crystallites. In neighboring grains, the crystal lattices are rotated relative to each other at a certain angle.

In crystallites, short- and long-range orders are observed. This means the presence of an ordered arrangement and stability as its closest neighbors surrounding a given atom (short order), and atoms located at significant distances from it up to the grain boundaries (long range order).

Metals are crystalline bodies, the atoms of which are arranged in a geometrically regular order, forming crystals, in contrast to amorphous bodies (for example, resin), the atoms of which are in a disordered state.

It should be noted that there is some difference between the concept of “metal” as a chemical element and as a substance. Chemistry divides all elements into metals and non-metals according to their behavior in chemical reactions. The theory of the metallic state considers large accumulations of metal atoms that have characteristic metallic properties: plasticity, high thermal and electrical conductivity, metallic luster. These properties are characteristic of large groups of atoms. Individual atoms do not have such properties.

Atoms in a metal are in an ionized state. Metal atoms, giving up some of their outer valence electrons, turn into positively charged ions. Free electrons continuously move between them, forming a mobile electron gas.

At room temperature, all metals, except mercury, are solids with a crystalline structure. Crystals are characterized by a strictly defined arrangement in space of the ions that form the crystal lattice.

Arranged in metals in a strict order, atoms in the plane form an atomic network, and in space - an atomic crystal lattice. Different metals have different types of crystal lattices. The most common lattices are body-centered cubic, face-centered cubic, and hexagonal close-packed.

The unit cells of such crystal lattices are shown in Fig. 1.2. The lines in these diagrams are symbolic; in reality, no lines exist, and the atoms vibrate near equilibrium points, i.e., lattice nodes, with a high frequency. In a cell of a cubic body-centered lattice, the atoms are located at the vertices of the cube and at the center of the cube; Chromium, vanadium, tungsten, molybdenum, etc. have such a lattice. In a cell of a cubic face-centered lattice, atoms are located at the vertices and in the center of each face of the cube; Aluminum, nickel, copper, lead, etc. have such a lattice. In a hexagonal lattice cell, the atoms are located at the vertices of the hexagonal bases of the prism, at the center of these bases and inside the prism; Magnesium, titanium, zinc, etc. have such a lattice. In a real metal, the crystal lattice consists of a huge number of cells.

The crystalline state is very common in nature: most solids (minerals, metals, plant fibers, proteins, soot, rubber, etc.) are crystals. However, not all of these bodies have the same clearly expressed crystalline properties discussed earlier. In this regard, bodies are divided into two groups: single crystals and polycrystals.

Monocrystal- a body, all the particles of which fit into one common spatial lattice. The single crystal is anisotropic. Most minerals are single crystals.

Polycrystal- a body consisting of many small single crystals, randomly located relative to each other. Therefore, polycrystals are isotropic, i.e.

Rice. 1.2. The main types of metal crystal lattices: A- cubic (1 atom per cell); b - body-centered cubic (2 atoms per cell);

V- face-centered cubic (4 atoms per cell); G- hexagonal close-packed (6 atoms per cell)

give the same physical properties in all directions. Metals are examples of polycrystals. However, a metal can also be obtained in the form of a single crystal if the melt is slowly cooled by first introducing into it one crystal of this metal (the so-called seed). A metal single crystal will grow around this embryo.

Depending on which particles the crystal lattice is formed from, there are four main groups of lattices: ionic, atomic, molecular and metallic.

Ionic lattice formed by oppositely charged ions held at lattice sites by electrical forces. The vast majority of crystals have an ionic lattice.

Atomic lattice formed by neutral atoms held at lattice sites by chemical (valence) bonds: neighboring atoms share external (valence) electrons. For example, graphite has an atomic lattice.

Molecular lattice formed by polar (dipole) molecules held at lattice sites also by electrical forces. However, for polar molecules the effect of these forces is weaker than for ions. Therefore, substances with a molecular lattice are relatively easily deformed. Most organic compounds (cellulose, rubber, paraffin, etc.) have a molecular crystal lattice.

Metal grate formed by positive metal ions surrounded by free electrons. These electrons bind the ions of the metal lattice together. This lattice is characteristic of metals.

Modern physics considers crystalline bodies to be solid bodies. Liquids, as already noted, are characterized by a random arrangement of particles, therefore liquids are isotropic. Some liquids can be greatly supercooled without becoming solid (crystalline). However, the viscosity of such liquids is so enormous that they practically lose their fluidity, retaining their shape, like solids. Such bodies are called amorphous. Amorphous bodies include, for example, glass, resin - rosin, etc. It is clear that amorphous bodies are isotropic. It should, however, be borne in mind that amorphous bodies can, over a long period of time, transform into a crystalline state. In glass, for example, crystals appear over time: it begins to become cloudy and turn into a polycrystalline body.

Amorphous state- a solid condensed state of matter, characterized by isotropy of physical properties due to the disordered arrangement of atoms and molecules. In addition to the isotropy of properties (mechanical, thermal, electrical, optical, etc.), the amorphous state of a substance is characterized by the presence of a temperature range in which the amorphous substance transforms into a liquid state with increasing temperature. This process occurs gradually: when heated, amorphous substances, unlike crystalline ones, first soften, then begin to spread and finally become liquid, i.e. amorphous substances melt over a wide temperature range.

Isotropy of properties is also characteristic of the polycrystalline state, but polycrystals have a strictly defined melting point, which makes it possible to distinguish the polycrystalline state from the amorphous one.

In amorphous substances, unlike crystalline ones, there is no long-range order in the arrangement of particles of the substance, but there is short-range order, observed at distances commensurate with the particle sizes. Therefore, amorphous substances do not form a regular geometric structure, representing structures of disordered molecules.

The structural difference between an amorphous substance and a crystalline one is detected using x-ray diffraction patterns. Monochromatic X-rays, scattered by crystals, form a diffraction pattern in the form of distinct lines or spots. This is not typical for the amorphous state.

Unlike the crystalline state, the amorphous state of a substance is not equilibrium. It arises as a result of kinetic factors and from a structural point of view is equivalent to the liquid state: an amorphous substance is a supercooled liquid with a very high viscosity. Typically, the amorphous state is formed during rapid cooling of the melt, when the substance does not have time to crystallize. This process is typical for the production of glasses, so the amorphous state is often called the glassy state. However, more often than not, even the fastest cooling is not fast enough to prevent crystal formation. As a result, most substances cannot be obtained in an amorphous state.

The spontaneous process of restructuring an amorphous substance into an equilibrium crystalline structure due to diffusion thermal displacements of atoms is practically endless. But sometimes such processes can be carried out quite easily. For example, after exposure to a certain temperature, amorphous glass “devitrifies,” i.e., small crystals appear in it and the glass becomes cloudy.

In nature, the amorphous state is less common than the crystalline state. It contains: opal, obsidian, amber, natural resins, bitumen. In an amorphous state, not only substances consisting of individual atoms and ordinary molecules, such as inorganic glasses and liquids (low-molecular compounds), but also substances consisting of long-chain macromolecules - high-molecular compounds, or polymers can be found. The physical properties of amorphous substances are very different from the properties of crystalline substances, due to which amorphous substances have found wide application in industry.

Polymers are widespread - organic amorphous substances, individual molecules of which, thanks to chemical (valence) bonds, are connected to each other (polymerized) into long chains, consisting in some cases of many thousands of individual molecules. Typical polymers are plastics. A very valuable property of polymers is their high elasticity and strength. Some polymers, for example, can withstand an elastic stretch of 2-5 times their original length. These properties of the polymer are explained by the fact that long molecular chains can, when deformed, curl into dense balls or, conversely, stretch into straight lines. Currently, polymers with specified properties (light, strong, elastic, chemically resistant, electrically insulating, heat resistant, etc.) are created from natural and artificial organic compounds.

Due to its maximum ordering, the crystalline state is characterized by minimum internal energy and is a thermodynamically equilibrium state under given parameters - pressure, temperature, composition (in the case solid solutions) etc. Strictly speaking, a completely ordered crystalline state cannot really be achieved; an approach to it occurs when the temperature tends to 0 K (the so-called ideal crystal). Real bodies in a crystalline state always contain some amount defects, violating both short- and long-range order. This is especially true in solid solutions, in which individual particles and their groups statistically occupy different positions in space.

Due to the three-dimensional periodicity of the atomic structure, the main features are homogeneity of properties and symmetry, which is expressed, in particular, in the fact that under certain conditions of formation, crystals take the form of polyhedra (see growth). Some properties on and near the surface of the crystal are significantly different from those properties inside the crystal, in particular due to symmetry breaking. The composition and, accordingly, properties change throughout the volume of the crystal due to the inevitable change in the composition of the medium as the crystal grows. Thus, the homogeneity of properties, as well as the presence of long-range order, refers to the characteristics of the “ideal” crystalline state

Most bodies in the crystalline state are polycrystalline and represent intergrowths of a large number of small crystallites (grains) - areas of the order of 10 -1 -10 -3 mm in size, irregular in shape and differently oriented. The grains are separated from each other by intercrystalline layers in which the order of the particles is disturbed. Concentration of impurities also occurs in the intercrystalline layers during crystallization. Due to the random orientation of the grains, the polycrystalline body as a whole (a volume containing quite a lot of grains) can be isotropic, for example, obtained with crystalline sedimentation. . However, usually in the process and especially in plastic, a texture appears - advantages, the orientation of crystal grains in a certain direction, leading to anisotropy of properties.

Due to the crystalline state, a single-component system can respond to several fields located in the region of relatively low temperatures and elevated ones. If there is only one field of the crystalline state and the substance does not chemically decompose with increasing temperature, then the field of the crystalline state borders on the fields of gas along the lines of melting and sublimation - condensation, respectively, and liquid and gas (vapor) can be in a metastable (supercooled) state in a field there is a crystalline state, whereas a crystalline state cannot be in a field or vapor, that is, a crystalline substance cannot be overheated above the melting or sublimation point. Some (mesogens) transform into a liquid crystalline state when heated (see. Liquid crystals). If there are two or more fields of the crystalline state on the diagram of a one-component system, these fields border along the line of polymorphic transformations. The crystalline substance can be overheated or supercooled below the temperature of the polymorphic transformation. In this case, the crystalline state under consideration may be in the field of other crystalline modifications and is metastable.

While liquid and vapor, due to the existence of a critical point on the evaporation line, can be continuously converted into each other, the question of the possibility of continuous mutual transformation of the crystalline state has not been finally resolved. For some substances, it is possible to estimate the critical parameters - pressure and temperature at which DH pl and DV pl are equal to zero, i.e., the crystalline state and liquid are thermodynamically indistinguishable. But in reality such a transformation was not observed for any of them (see. Critical condition).

A substance can be transferred from a crystalline state to a disordered state (amorphous or glassy), which does not meet the minimum free energy, not only by changing state parameters (pressure, temperature, composition), but also by exposure to ionizing radiation or fine grinding. The critical particle size, at which it no longer makes sense to talk about the crystalline state, is approximately 1 nm, i.e. of the same order as the size of the unit cell.